The present invention relates to improved compositions which cure by way of a precious metal-catalyzed reaction of silicon-bonded radicals and have an improved bath life. The present invention further relates to an improved process for using said compositions.
Organosilicon compositions in which a platinum group metal-containing catalyst is inhibited in its cure-promoting activity at room temperature by the presence of a catalyst inhibitor are well known in the organosilicon art and need no detailed discussion herein.
Among the many inhibitors of the art the maleate inhibitors of U.S. Pat. Nos. 4,256,870 and 4,562,096, and the fumarate inhibitors of U.S. Pat. No. 4,774,111, are of particular concern.
The maleates have been found to be particularly effective for increasing the room temperature bath life, i.e. work time, of solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction. However, the heating time and/or temperature needed to cure these maleate-inhibited compositions are/is sometimes excessive. When one attempts to decrease the cure time and/or temperature of these compositions to a commercially desirable interval by using less maleate and/or more catalyst the bath life is frequently decreased to a commercially undesirable interval.
The fumarate inhibitors have been found to allow a cure of a solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction to take place at a suitable heating time and/or temperature. However, the bath life of such a composition, as measured by gel time at room temperature, is not as long as desired. When one attempts to increase the bath life of these compositions by increasing the amount of fumarate and/or decreasing the amount of catalyst the cure time and/or temperature increases.
This problem of increased cure time and/or cure temperature with increased bath life in an inhibited platinum group metal-catalyzed system is of particular significance for applications where the organosilicon composition is used to rapidly coat a substrate over a long period of time. In such a process a long bath life coupled with a short cure time, preferably at low-to-moderate temperature, is an essential property of the coating composition.
In the coating arts, such as the paper coating art, the coating composition that is used to coat a substrate should not cure to the extent that its viscosity has increased substantially before it has been applied to the substrate; however, it should rapidly cure thereafter, preferably with only a moderate amount of added energy. Typically this means that the coating compositions preferably should not gel for as long as eight hours but should cure rapidly at moderately increased temperature to such an extent that the coated substrate can be further processed, if desired, without damaging the coating. In addition, the cure time of the composition at a given cure temperature desirably should remain substantially constant as the bath ages.
In the preparation of laminates comprising a peelable release paper bearing a cured coating and an adhesive film releasably adhered thereto, such as a stick-on label, one of two processes is normally used. In one process, the off-line process, the silicone composition is coated on paper and cured; then, at a later time, an adhesive film is applied to the cured silicone coating. In the other process, the in-line process, the silicone composition is applied to paper and cured and the adhesive is then immediately coated on the cured silicone coating. While the in-line process is generally more efficient and would normally be more advantageous, it has been found that some adhesives bond, i.e. weld, to the silicone coated paper if the adhesive is applied to the silicone coated paper within a short time after the silicone composition has been cured. It is believed that residual reactivity in the cured silicone coating is responsible for the welding of the silicone and adhesive in the in-line process.
While the art has proposed and provided some solutions for the welding problem there is a need for further improvements in a release-coating composition for in-line lamination of adhesives which cures at lower temperature and/or has a longer bath life.
There is also interest in applying silicone release coatings to substrates, such as polyethylene sheets, which are less stable at the elevated temperatures used to cure the silicone release coatings of the art. Consequently there is a need for silicone coatings which cure at reduced temperatures. Of course, silicone coatings which have lower curing temperatures or faster curing rates are also desired for energy conservation and for more efficient production processes.